1. Field of the Invention
The invention relates to sand molding, and more particularly to modern, automatic, repetitive, high-speed molding processes.
2. Description of the Prior Art
A well known sand molding process consists of forming essentially parallelopiped blocks by compressing sand in a horizontal press between a front pattern plate which opens and a back pattern plate which exerts pressure. In this process, the blocks are assembled into a continuous horizontal column as they leave the press, forming a complete mold impression at each joint between blocks, which will then enable metal to be poured repeatedly into the various impressions. The blocks continue to advance, while the pieces cool, up to an end station where the sand is removed. In each operating cycle, sand is projected vertically and at high speed into the press between the closed front plate and the withdrawn back plate before the pressure is applied.
The sand used is molding sand called green sand, a material composed of silica grains mixed with a clay called bentonite, powdered charcoal, and water. By appropriate mixing, each solid grain is coated with a thin film of suitably hydrated clay in order to impart the necessary adhesive power under a high forming pressure, generally on the order of 10 kg/cm.sup.2.
Like any molding sand, this sand must meet conflicting requirements, since it must be sufficiently fluid to be able to be easily packed around all of the details of the pattern plates, while at the same time being sufficiently stiff not to collapse after packing nor to lose shape after separation from the pattern plates. It must also exhibit good cohesion, particularly a shearing resistance high enough to resist the stresses of molding, such as the hydrostatic pressure of molten metal, the thermic shocks at the moment of pouring, and the stresses due to the contraction of the metal in the process of solidification. Moreover, it must be able to be broken for the extraction of the pieces without too much difficulty.
Modern molding processes such as the one described above are even more demanding on the quality of the sand due to the absence of any frame, which thus necessitates a greater self-cohesion of the blocks, the high pressure exerted on the sand in the joint planes of the lead blocks, which must push back the whole column and, finally the fact that the sand removal operation itself must be automatic and take place at high speed. In each case, an optimal quality of sand must be set and it is necessary to monitor this quality to ensure that it remains within the narrow and precise limits thus defined.
The usual methods for monitoring the quality of the sand consist of drawing off a sample of non-packed sand, i.e., from the hopper of the machine, forming a test cylinder (5 cm.times.5 cm) in an outside laboratory apparatus, compressing it at 10 kg/cm.sup.2, and shearing it on a special machine. Depending on the result of this measurement, the replenishment of agglomerating agents, or bentonite clay, is adjusted as a function of the observed fluctuations from the set limits. These measurements are repetitive, but as a result of the process used the measurement speed is always slow and rarely exceeds more than two measurements per hour, which nevertheless requires a full-time operator and imposes a great lag between the moment of sampling and the moment at which the result of the measurement is finally known.
This known process permits the above demand for constancy of sand quality to be met only in a way that is approximate. In addition, the check covers only the quality of the sand per se, prior to packing, and not the real conditions in which it is found inside the block.